Thin-film transistors and other electronic devices using organic semiconductors are emerging as alternatives to established methods using amorphous silicon (α-Si:H) as the semiconductor.
A variety of organic compounds have been proposed and tested as semiconducting materials for TFT devices. For example, among the p-channel (hole transport) materials that have been characterized are thiophene oligomers proposed as organic semiconductor material for TFT in Garnier, F., et al., “Structural basis for high carrier mobility in conjugated oligomers” Synth. Meth., Vol. 45, p. 163 (1991), and phthalocyanines described in Bao, Z., et al., “Organic Filed-effect transistors with high mobility based on copper phthalocyanine” Appl. Phys. Lett., Vol. 69, p. 3066 (1996). Pentacene, which is a member of poly(acene) compounds has been proposed as an organic semiconductor material in Lin et al. IEEE 54th Annual Device Research Conference, 1996, pages 2136–2139, and Dimitrakopoulos et al., J. Appl. Phys., 80 (4), 1996, pages 2501–2507.
Some soluble organic compounds have also been characterized as organic semiconducting materials. For example poly(3-alkylthiophene) described in Bao, Z., et al., “Soluble and Processable regioregular poly(3-hexylthiophene) for thin film field-effect transistors application with high mobility” Appl. Phys. Lett., Vol. 69, page 4108 (1996).
An attractive material would have a high carrier mobility which is close to that of amorphous silicon (0.1–1 cm2.V−1.Sec−1), with a very high on/off ratio (>105). For an organic material to replace amorphous silicon, not only would it have the electrical properties cited above but also it should be processable from solution so that it could become commercially attractive.
Among the organic compounds which have been studied as semiconductors, only regioregular poly(3-hexylthiophene) is readily soluble in organic solvents and thin films of this compound has been processed from solution for construction of TFTs. The drawback for this compound is that it has relatively low (5×10−2 cm2.V−1.Sec.−1) carrier mobility and even much less satisfactory on/off ratio of less than 100. In addition, because thin films of this polymer are not stable in air and its field-effect characteristics deteriorates on exposure to air, its application as semiconductor becomes less desirable.
The best performance as semiconductor among organic materials to date has been achieved by thin films of pentacene deposited under high vacuum and temperature as reported by Dimitrakopoulos et al., in U.S. Pat. Nos. 5,946,511; 5,981,970 and 6,207,472 and other publications by Brown et al., J. Appl. Phys. 80(4), 1996, pages 2136–2139 and Dimitrakopoulos et al., J. Appl. Phys. 80 (4), pages 2501–2507.
Recently, thin-film transistors on plastic substrates using evaporated films of pentacene as the p-channel carrier with mobility of 1.7 cm2.V−1.Sec.−1 and an on/off ratio of 108 has been reported by Jakson et al., in Solid State Technology, Vol. 43 (3), 2000, pages 63–77.
Thin films of pentacene are very stable in air and even moderate temperatures and as far as performance is concerned, pentacene is the most attractive organic material to replace amorphous silicon.
The drawback for pentacene is that it is insoluble in common organic solvents and it can only be deposited as thin film by expensive high vacuum and temperature techniques.
There has been very little effort for the synthesis of soluble pentacene derivatives and the only example of soluble pentacene is by Muellen, K. et al., “A soluble pentacene precursor: Synthesis, solid-state conversion into pentacene and application in a field-effect transistor,” Adv. Mat. 11 (6), p. 480 (1999), in which a precursor of pentacene is synthesized by a tedious multi-step synthetic approach. The derivative, which is soluble in organic compounds and can be processed from solution, is converted back to pentacene by heating at moderate to high temperature (140–200° C.).
The drawback for using this compound as a pentacene precursor is that due to multi-step synthesis (more than 9 steps), its preparation, especially in large scale is almost impractical. In addition, its conversion to pentacene occurs at a relatively high temperature, which prevents the use of most plastics as substrates.
Thioaldehydes, RCHS, and the more stable thioxoacetate ROCO—CHS have been used in Diels-Alder reactions with variety of dienes as reported by G. Kirby et al., in “Ethyl and methyl thioxoacetates, dienophilic thioaldehydes formed from sulphenyl chlorides by 1,2-elemination,” J. Chem. Soc., Perkin Trans, 1, 1541 (1985).
The adduct of anthracene which is the second member of polyacene with thioxoacetate and its oxidation to less stable S-oxide was reported by G. Kirby et al. in “Generation of a thioaldeyde S-oxide by retro-Diels-Alder reaction”, Chem. Commun., 718, 1987. This reaction, shown below, is an example of a Diels-Alder adduct as a precursor of fused aromatic compounds.

Another example of hetero dienophile with one hetero atom are oxomalonates and thioxomalonates (RR′C═O and RR′C═S respectively) as shown by J. Barluenga et al., in “Diels-Alder reaction of unactivated 2-Aza-1,3-dienes with diethyl ketomalonate: A carbon dioxide equivalent” Tetrahedron Lett., Vol. 30, pages 2685–2688, 1989 and G. Kirby et al., in “The transient dienophile and its S-oxide (sulphine) formed by retro Diels-Alder reaction”, J. Chem. Soc., Perkin Trans., 3175, 1990 for oxo- and thioxomalonate respectively. Again, one of the dienes shown to react with this hetero dienophile, is anthracene in which a labile anthracene adduct having carbon-sulfur bond is formed as shown in the following scheme:

Hetero Diels-Alder reactions with dienolphiles having two active heteroatoms have also been used successfully for the preparation of a variety of nitrogen and/or sulfur containing compounds. An example of such dienophiles are acylnitroso (RCO—N═O) compounds which are generated in situ from corresponding hydroxamic acids. An extensive review of Diels-Alder reactions of various dienes with acylnitroso dienophiles has been published by M. Miller et al., in “Development and applications of amino acid derived chiral acylnitroso hetero Diels-Alder reaction”, Tetrahedron, Vol. 54, pages 1317–1348, 1998. Again, the use of anthracene or its derivatives has been a prime example in this report as shown in the following scheme:

Other hetero dienophiles with two heteroatoms which have been used in Diels-Alder reactions are those with —N═N— and R—N═S═O functionalities. Examples of dienophiles with nitrogen-nitrogen double bonds are various azodicarboxylates (ROCO—N═N—COOR) and those with nitrogen-sulfur double bonds are N-sulfinyl amines or N-sulfinyl amides (R—N═S═O) or (RCO—N═S═O).
None of the above references describes precursors of polycyclic aromatic compounds that are: (1) Diels-Alder adducts of a polycyclic aromatic compound, such as, pentacene, with a dienophile and (2) highly soluble in common organic solvents.
Accordingly, it is an object of this invention to synthesize precursors of polycyclic aromatic compounds, such as, pentacene, which are highly soluble in common organic solvents.
Another object of this invention is to provide a precursor of a polycyclic aromatic compound, such as, pentacene, that is convertible in bulk or as thin films back to pentacene in a retro-Diels-Alder reaction at relatively low temperatures.
Still another object of the invention is to devise a simple synthetic approach to the preparation of these compounds, which produces high yields of the compounds and is easily scalable.
Still another object of this invention is to cast thin films of these materials from solution and regenerate thin films of pentacene upon heating the former at low or moderate temperatures.
The present invention provides highly soluble precursors of polycyclic aromatic compounds, such as, pentacene, which are synthesized in one step via the Diels-Alder reaction of polycyclic aromatic compound with a variety of dienophiles having at least one heteroatom in the dienophile moiety.